We are studying murine leukemia from several aspects. In vitro lymphoma cell lines are established from spontaneous thymomas and Gross virus accelerated lymphomas in AKR mice. Their virus expression is evaluated using the XC plaque assay and the acceleration of lymphoma in AKR mice (oncogenicity). Virus host range is also determined. Virus expression in several tissues of AKR mice of different ages is also studied, using these parameters. A third area of study centers around a system of immunoprophylaxis of a highly malignant syngeneic Gross virus induced lymphoma using endogenous AKR murine leukemia virus as the immunogen. These studies have demonstrated at least four types of AKR viruses; 1) XC plus, non oncogenic, ecotropic, 2) XC minus, non-oncogenic, xenotropic, 3) XC minus, oncogenic ecotropic, 4) XC minus, oncogenic, dual host range. These viruses vary in their expression in each lymphoma cell line, and at different times in the life of the animal. They also differ in their ability to prevent in vivo growth of the syngeneic lymphoma.